SAN FRANCISCO - Facebook has quietly expanded the availability of technology to automatically identify people in photos, renewing concerns about the privacy practices of the world's top social networking service.

The feature, which Facebook automatically enabled for Facebook users, has been expanded from the United States to "most countries", Facebook said on its official blog on Tuesday.

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Its "Tag Suggestions" feature uses facial recognition technology to speed up the process of labeling friends and acquaintances that appear in photos posted on Facebook.

The company's rollout of the technology has raised eyebrows in some circles. Internet security consultant firm Sophos published a post on its company blog on Tuesday saying that many Facebook users are reporting that the site has enabled the facial recognition option in the last few days without giving users any notice.

"Yet again, it feels like Facebook is eroding the online privacy of its users by stealth," wrote Graham Cluley, a senior technology consultant at Sophos.

Facebook, which announced in December that it planned to introduce the service in the United States, acknowledged on Tuesday that the feature was in fact now more widely available.

When asked about the Sophos blog post, Facebook said in an emailed statement that "we should have been more clear with people during the roll-out process when this became available to them."

The statement noted that the photo-tagging suggestions are only made when new photos are added to Facebook, that only friends are suggested and that users can disable the feature in their privacy settings.

The company did not respond to requests for further comment.

While other photo software and online services such as Google Inc's Picasa and Apple Inc's iPhoto use facial recognition technology, the use of the technology on an Internet social network like Facebook, which counts more than 500 million users, could raise thorny privacy issues.

Marc Rotenberg, President of the non-profit privacy advocacy group Electronic Privacy Information Center, noted that Apple's iPhoto software gave users control over facial recognition technology by letting them elect whether or not to use the technology with their personal photo collections.

Facebook's technology, by contrast, operates independently, analyzing faces across a broad swathe of newly uploaded photos.

Rotenberg said such a system raised questions about which personally identifiable information, such as email addresses, would become associated with the photos in Facebook's database. And he criticized Facebook's decision to automatically enable the facial-recognition technology for Facebook users.

"I'm not sure that's the setting that people would want to choose. A better option would be to let people opt-in," he said.

Last year the Electronic Privacy Information Center filed a complaint about Facebook's privacy practices with the US Federal Trade Commission, which Rotenberg said was still pending. He noted that he planned to take a close look at Facebook's new announcement involving facial recognition technology.